About almost all of display apparatuses, the displaying performance thereof is changed in accordance with viewer's viewing angle. A typical example of the apparatuses is a liquid crystal display apparatus, a typical mode of which is a twisted nematic (TN) mode.
The wording “being changed in accordance with viewer's viewing angle” means that between the case of observing a display apparatus from the front surface direction (in the direction of a normal line (normal direction) of an observing surface of the display apparatus, i.e., a direction of a viewing angle of 0°) and the case of observing the display apparatus from an oblique direction (in a direction of a viewing angle more than or less than 0°), a difference is generated in displaying performances such as contrast ratio, gradation property, and chromaticity. It is known that these displaying performances are generally poorer when the display apparatus is observed in the oblique direction than when observed from the front surface direction.
Display apparatuses are required to have various displaying performances, for example, such a performance that the viewing angle is enlarged while the apparatuses maintain a bright display, and such a performance that the apparatuses are decreased in color change with the enlargement in the viewing angle.
The wording “viewing angle” denotes the following angle when the front surface direction of a display apparatus (normal direction of an observing surface of the display apparatus, or a direction of a viewing angle of 0°) is regarded as 0°: an angle, within a range of −90 to 0° and one of 0 to +90°, at which an viewer or observer views the display apparatus. The reason why the value of the viewing angle may be minus is that when any value in one of the ranges is regarded merely as a plus value, any value in the other is regarded as a minus value. Thus, the reason is a convenient reason. As the absolute value of this viewing angle increases, the display apparatus is generally decreased in luminance. In a flat panel display (FPD) such as a liquid crystal display apparatus, for a structural reason thereof, as well as for a nature that light diffuses more easily as the wavelength thereof is made shorter, a problem remains that when the absolute value of the viewing angle is increased, the resultant image is disturbed in balance between colors to be more easily changed in color.
However, in conventional techniques, neither an effect against the problem that the luminance is lowered as the absolute value of the viewing angle is increased, nor an effect of decreasing the color change with an increase in the absolute value of the viewing angle is sufficient.
In one of the conventional techniques, it is conceived that a member having an isotropic light-diffusing property (for example, see Patent Literature 1) is used in a display apparatus. A mechanism that this light diffusion member expresses light diffusion is classified into light scattering based on irregularities formed in a surface (light surface scattering), light scattering based on a difference in refractive index between a matrix resin and fine particles dispersed therein (light internal scattering), and light scattering based on both of light surface scattering and light internal scattering.
However, the light diffusion film described in Patent Literature 1 is used in a plane light source; thus, when this film is used for an observing surface of a display apparatus, viewer's viewing angle is somewhat enlarged, but it is very difficult that the display apparatus is increased in viewing angle absolute value while maintaining a bright display. Thus, a difficult problem remains in order to decrease a color change with the increase in the viewing angle absolute value. Moreover, the above-mentioned light diffusion member is merely a member having a property of isotropic diffusion, and has a problem that a blurred image is easily generated.
In the meantime, as a member different in optical property from the light diffusion member, known is a light control plate or an anisotropic diffusion medium, which intensely diffuses incident light having an angle in a predetermined angle range but transmits incident light having an angle out of the range. As illustrated in FIG. 6, the light control plate has therein tabular structures (for example, Patent Literature 1). As illustrated in FIG. 7, the anisotropic diffusion medium has therein columnar structures (for example, Patent Literature 2).
It can be checked by a method illustrated in FIG. 8 that such a member is varied in diffusing property (indicates anisotropy) in accordance with the incident angle (of light thereto). As illustrated in FIG. 8, a sample is arranged between a light source not illustrated, and a light receiving unit 3. While the angle of the sample is varied around a central axis that is a straight line L on the surface of the sample, light is straightly transmitted through the sample. The linear transmittance of the light radiated into the light receiving unit 3 is measurable.
FIG. 9 shows the incident angle dependency of the scattering property that the light control plate 50 illustrated in FIG. 6 has, the dependency being measured using the method illustrated in FIG. 8. Its vertical axis represents the linear transmittance (of the film) (i.e., at the time that parallel rays having a predetermined light quantity are incident, light quantity of emitted-out parallel rays in a direction identical with the incident direction), the linear transmittance being an index showing the degree of scattering (on the sample). Its horizontal axis represents the incident angle of the rays. In FIG. 9, a solid line and a broken line show cases of rotating the light control plate 50, respectively, around the center of an axis A-A (penetrating the tabular structures) and around the center of an axis B-B (parallel with the tabular structures) in FIG. 6. It is understood that by changing the axis of the light control plate 50, the plate 50 is changed in properties, as illustrated in FIG. 9.
FIG. 10 shows the incident angle dependency of the scattering property that the anisotropic diffusion medium illustrated in FIG. 7 has, the dependency being measured using the method illustrated in FIG. 8. It is understood that even when changing the axis of the anisotropic diffusion medium, the properties of the medium 1 are not changed very much, as illustrated in FIG. 10.